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We show that  Yersinia enterocolitica  strain Ye9 (bio-serotype 2/O:9) utilizes heme-containing molecules as an iron source. The Ye9 genome contains two multigenic clusters,  hemPRSTUV -1 and  hemPRST -2, encoding putative heme receptors HemR1 and HemR2, that share 62% amino acid identity. Expression of these proteins in an  Escherichia coli  mutant defective in heme biosynthesis allowed this strain to use hemin and hemoglobin as a source of porphyrin. The  hemPRSTUV -1 and  hemPRST -2 clusters are organized as operons, expressed from the p hem−1  and weaker p hem−2  promoters, respectively. Expression of both operons is negatively regulated by iron and the iron-responsive transcriptional repressor Fur. In addition, OmpR, the response regulator of two component system (TCSs) EnvZ/OmpR, represses transcription of both operons through interaction with binding sequences overlapping the −35 region of their promoters. Western blot analysis of the level of HemR1 in  ompR, fur , and  ompRfur  mutants, showed an additive effect of these mutations, indicating that OmpR may regulate HemR expression independently of Fur. However, the effect of OmpR on the activity of the p hem−1  promoter and on HemR1 production was observed in both iron-depleted and iron-replete conditions, i.e., when Fur represses the iron-regulated promoter. In addition, a hairpin RNA thermometer, composed of four uracil residues (FourU) that pair with the ribosome-binding site in the 5′-untranslated region (5′-UTR) of  hemR1  was predicted by  in silico  analysis. However, thermoregulated expression of HemR1 could not be demonstrated. Taken together, these data suggest that Fur and OmpR control iron/heme acquisition via a complex mechanism based on negative regulation of  hemR1  and  hemR2  at the transcriptional level. This interplay could fine-tune the level of heme receptor proteins to allow  Y. enterocolitica  to fulfill its iron/heme requirements without over-accumulation, which might be important for pathogenic growth within human hosts.

OmpR-Mediated Transcriptional Regulation and Function of Two Heme Receptor Proteins of Yersinia enterocolitica Bio-Serotype 2/O:9